There has been proposed a method for detecting a thickness of a magnetic tape in which reel periods are determined at different winding states of a successive magnetic tape wound on supplying and winding reels and an operation is made on the determined reel periods.
As shown in FIG. 3, a winding radius Rs of a supplying reel 1 before a magnetic tape 3 begins to be transferred from the supplying reel 1 to a winding reel 2 can be determined by a ratio of rotational numbers of the respective reels when a predetermined length of the magnetic tape is transferred, if the winding radius Ro of the winding reel 2 is known.
However, the prior art method has a disadvantage in which the reel periods at the different winding states for determining the tape thickness should be detected under a condition where the tape transferring velocities at the respective winding states are equal to each other.
Furthermore, when the winding radius Rs of the supplying reel before the magnetic tape begins to be transferred is to be determined, the precision of a ratio of the rotational numbers of the respective reels depends upon a number of pulses for detecting the respective reel rotational states. Therefore, many rotational numbers of the reels are required. However, with so many rotational numbers of the reels to be detected, the tape winding radius of the respective reels tends to vary during the detection thereof. As a result, the detected ratio of the rotational numbers of the reels corresponds to a ratio of average values of the winding radius of each of the reels varying during the pulse detection. Thus, it will be noted that the winding radius Rs of the supplying reel before the magnetic tape begins to be transferred cannot be precisely determined.